The invention relates to a pneumatic hammer of the kind mentioned in the precharacterizing part of claim 1.
Such pneumatic hammers are used for ground or rock drilling. They may be implemented in connection with drilling machines that advance and rotate drill rods with a drill bit from a boring frame. In this case, the pneumatic hammer is generally designed as a in-hole hammer which is arranged immediately behind the drill bit in the drill rods. Further, pneumatic hammers may be designed as hand-held hammers, so-called compressed air hammers, which are operated by hand in order to do demolition work or ground and rock work. With a hand-held hammer, the drill bit generally is a simple trepan.
In pneumatic hammers with pin drill bits, the impact energy supplied by the working piston is transmitted to the hard metal pins or bezels for cleaving rock via the drill bit. The impact frequency is determined by the quantity of compressed air supplied or by the quantity transmitted by the pneumatic hammer. By rotating the entire drilling tool, the bottom of the bore hole is cleft and stripped and the drilling material is transported to the outside by the relaxing and outflowing discharge air in the annular gap between the drill rod and the inner wall of the drill rod.
The drilling capacity is chiefly determined by the following factors:
the single impact energy imparted on the drill bit by the working piston during every blow; PA1 the number and the surface of the drill bit pins on which the impact energy is distributed and which transform that energy into penetration and cleaving work; PA1 the impact frequency; PA1 the pressure of the drilling tool on the bottom of the bore hole; PA1 the removal of the drillings or the purging or rinsing of the bottom of the bore hole to clean the same of the drillings.
The drive energy required for pneumatic hammers is supplied by compressors. Normally, the supply pressure is about 7 to 10 bar and the supply quantity is about 5 m.sup.3 /min.
Recently, high pressure compressors are used on building sites that supply a pressure in the magnitude of 20 bars. Such high pressure compressors are also used to drive the pneumatic hammers used on a building site, even if these pneumatic hammers were originally designed for pressures between 7 and 10 bars. For such high pressure operation, the principle of these pneumatic hammers has not been changed; only certain components of the hammer have been provided with a greater strength or a greater thickness. This results in the same pneumatic hammers being operated in a wide range of supply pressures between 7 and 25 bars. With a higher supply pressure, the impact frequency and the impact energy will increase, but the drilling capacity is not enhanced correspondingly. This is due to the fact that the impact energy per drill bit pin is essential for the drilling capacity. The drilling capacity will only be optimal, if the impact energy per drill bit pin is maintained in a certain range. Above this range, the cleaving depth of the rock (cleaving work) is not substantially improved, although the consumption of compressed air increases vastly. Thus, the actual drilling capacity is far behind the installed power of the compressor, which results in a low efficiency. Additionally, a high impact energy of the working piston generates a jarring blow on the anvil. Such jarring blows cause an enormous stress on the drill bit shaft and the working piston, often resulting in ruptures of shafts and pistons. In manually operated pneumatic hammers, the jarring blows caused by an excessive supply pressure entail serious physical stresses on the operator, including the risk of detrimental effects on his health and in particular on the skeletal structure.
The operator of a drilling device will usually obtain the drilling tools, the compressor, the pneumatic hammer and the drill bit from different manufacturers, respectively. As a rule, this leads to an untuned combination of elements being implemented. The operator is not able to select the components such that an optimal drilling capacity with a high efficiency can be obtained with a simultaneous low stress on the material.